Back in December, we addressed questions raised over Jason Bartlett’s sudden decline in defensive performance in 2008. At the time we speculated that in addition to the two injuries he played through during the season, playing with a gold glove-caliber third baseman may have directly impacted Bartlett’s numbers.

Using Baseball Musings’“Probabilistic Model of Range,” we reasoned that the addition of Evan Longoria, who displays great range to his left (towards second base) may be taking away opportunities from Bartlett due to a significant overlap in their defensive range.

Now Baseball Prospectus (via Drays Bay), using a defensive plus/minus metric, shows us that Bartlett was just as effective moving to his right in 2008 and that his defensive decline was due entirely to fielding chances to his left.

…John Dewan’s other defensive number, Plus/Minus, had Bartlett at -1 in 2008 after seasons of +13 and +18 in 2006-2007. That’s a massive and sudden degradation, made even stranger since Bartlett was just as effective going to his right both years (+11); it was when going to his left (a drop from +4 to -11) that his defense became a problem for him in 2008.

So despite the significant overlap in range, Bartlett does not appear to have been impacted by the improved range of his third base partner.

Now it appears that his defensive decline was due entirely to his shoulder and leg injuries. While possible, it does not seem likely that a player’s defensive skills would erode so quickly. It may be that one or both injuries was more troublesome when Bartlett was moving to his left. The throwing motion alone, for a shortstop moving to his left, is considerably different and may have been difficult or painful for Bartlett.

Whatever the explanation, a healthy Bartlett in 2009 should go a long ways towards once again being one of the premiere defensive shortstops in baseball.